In recent years, significant development work has been done relative to non-impact printing systems, specifically ink jet printing. There are several types of ink jet printing, including drop on demand systems, magnetic ink jet systems and electrostatic pressurized ink jet. In each of the systems, the accuracy in printing is related to the directional control over the ink jet droplets. In the systems where only a single ink jet is involved, any initial misdirection of the jet may be corrected by adjusting the aim of the jet nozzle or by biasing directional control over the ink jet drop stream. In multiple jet systems, space considerations may prevent individual control over each jet. Further, the initial directionality may be altered as a result of dried ink on the nozzle, partial clogging of the nozzle, or by wear of the nozzle. It is therefore necessary that the jet directionality be checked, not only when the nozzle is first placed in the machine for operation, but periodically.
Referring, for example, to multiple-nozzle electrostatic pressurized ink jet, conductive ink is supplied under pressure to an arrangement of closely spaced nozzles. The ink is thus propelled from each nozzle in a stream which is caused to break up into a train of individual droplets which must be selectively charged and controllably deflected for recording or to a gutter. Such a system is described in U.S. Pat. No. 3,373,437 of Richard G. Sweet et al., titled "Fluid Droplet Recorder with a Plurality of Jets". In such electrostatic systems, the drop charging occurs at a charging electrode at the time that the drop breaks off from the ink jet stream. The drop will thus assume a charge determined by the amplitude of the signal on the charging electrode at the time the drop breaks away from the ink jet stream. The drop thereafter passes through a fixed electrical field and the amount of deflection is determined by the amplitude of the charge on the drop at the time it passes through the deflecting field. In the binary type of electrostatic ink jet, such as described in Sweet et al., above, uncharged drops are not deflected and proceed directly to a recording surface positioned downstream from the deflecting means such that each such drop strikes the recording surface and forms a small spot. The deflected drops deviate from the uncharged drop path a sufficient amount such that they are intercepted by a catcher or gutter apparatus.
If the directionality of the jet stream prior to charging or if the timing of the drop breakoff relative to the charging signal are not precisely correct so that the drop will not be completely charged, the drop may be deflected an insufficient amount to be completely intercepted by the drop catcher or gutter. The drop or splatter from the drop may thus impact the recording medium.
Further, should the initial directionality of the jet stream be incorrect, the resulting spots on the recording surface would be improperly aligned.
It is therefore necessary to periodically test the directionality of the ink jet stream, whether charged or uncharged. Various systems have been developed to detect the charge synchronization of electrostatic ink jet drops, i.e., whether the drops are fully charged and thus synchronized with the charge signal. Some systems are further arranged to detect the directionality of charged drops. Examples are as follows: U.S. Pat. No. 3,852,768 of John M. Carmichael et al. entitled "Charge Detection for Ink Jet Printers" and U.S. Pat. No. 3,886,564 of Hugh E. Naylor et al. entitled "Deflection Sensors for Ink Jet Printers", both of which disclose induction sensors which may detect deflected directionality by placing the sensor at a position by which the drops of charged ink are to pass if properly charged and deflected; U.S. Pat. No. 3,898,673 of John W. Haskell entitled "Phase Control for Ink Jet Printer" discloses a multi-section gutter having a pair of contacts in one or more of the gutter sections to sense the conductivity increase when electrodes are wetted by a number of the electrostatic ink jet droplets; and U.S. Pat. No. 3,465,350 of Robert I. Keur et al. entitled "Ink Drop Writing Apparatus" describes the use of a piezoelectric member which generates a signal in response to drop impact anywhere on the member.
The induction sensors above give low amplitude signals which are sensitive to noise, are dependent upon the level of charge, and are not suitable for uncharged drops. The conductivity sensor senses the wetting of a specific area without giving specific locations within the area, and is limited to electrically conductive ink. The piezoelectric impact transducer gives a weak output signal in response to the pressure of successive drops falling anywhere thereon, and does not give specific location information.
It is therefore an object of the present invention to provide a sensing apparatus which gives precise location information without regard to the nature of the drops whose location is sensed.